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New Paradigms in Magnetic Recording

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					             New Paradigms in Magnetic Recording:
         Understanding through Micromagnetic Modeling.
         i l
     Martin Plumer. Department of Physics and Physical Oceanography, Memorial University.
                              (Design engineer, Seagate Technology 1997 - 2005)
1.     Overview of magnetic recording:
     - Head (Write Element & Read Element-GMR spin valve) and Recording Medium (disc).
     - Fundamental Limitations: Superparamagnetism (a Bad thing).

2. Micromagnetics:                   
                                                        
                                                                            
                                 2 dM
                                                             
                           (1   )     M  H eff     M  M  H eff           LLG equation
                                    dt                 M
          P bl
      THE Problem: Long-Range Magnetic Dipole Interactions
                                    i i l             i

3. Micromagnetic Recording Model: Writer/Reader/Media
     - Impact of Recording Medium effects (disc microstructure) on the
       quality of recorded bits.

   What s
4. What’s New: Perpendicular and Tunneling.
5. And What’s Next ?
                                             Reduction in growth rate due to
                                             superparamagnetism


2006
                                                                              Self Organized
                                                                              Magnetic Arrays
                                                                Heat Assisted Magnetic
                                                                Recording

                                               40%


                                      100%




AD=(1/track width)*(1/bit length)
  =(tracks per inch)(bits per inch)
• Today: AD ~ 300Gb/in2 Bit
                                                            Eric Meloche
  length ~ 3 media grains ~ 200 Å                           February 14, 2008
• Tomorrow: AD ~ 1000Gb/in2. Bit
  length ~ 1 media grain ~ 50 Å
     Press Release.               Source: Western Digital Technologies
WD(R) Launches Industry s First 2 TB Hard Drives
Tuesday January 27, 2009.

WD's Eco-friendly, Cool and Quiet, WD Caviar® Green(TM) Drive Marks the Largest
Capacity Hard Drive in the Industry

LAKE FOREST, Calif. Jan. 27 /PRNewswire-FirstCall/ -- WD (NYSE: WDC - News) today
                                                     world's
announced the first 2 terabyte (TB) hard drive - the world s highest capacity drive and the
latest addition to WD's popular, environmentally friendly, cool and quiet, WD® Caviar®
Green(TM) hard drive family. This new 3.5-inch platform is based on WD's industry-leading
500 GB/platter technology (with 400 Gb/in2 areal density) with 32 MB cache, producing
drives with capacities of up to 2 TB.
The three elements of magnetic recording.                                 1. Overview
                                                                      a
                          GMR Read Inductive
  Giant Magneto-Resistive          Write Element
                          Sensor


 Longitudinal
 Recording.                                                                    Media
                                                         d
                                                                       D=      grains
                                               NiFe
         W                               GAP
                                                                     8-10 nm

            N       S S   N N   S S   N N   SS       N N    S

                 B         Recording Medium           One ‘bit’ of information

   AD=(bpi)(tpi)
   AD (b i)(t i)
   bpi ~ 1/B
   tpi ~ 1/W
  Recording Head
                                                                        1. Overview
     X-section and ABS view of an Integrated MR Head


            media                      SEM image

                    Top pole

                             Coils                                      X
write gap
                          WRITER
Magneto             Return pole
                                                             Top pole
Resistive                                                                gap
(MR)                shared pole
                    Bottom shield                  shields
reader
element                                                  View from the air
                                                         bearing surface (ABS).
   ABS: Air Bearing Surface
                                                                        Read Sensor
                                    ~ 10 m
Finite Element Method: Magsoft                           2. Ferromagnets
                                                      2. Micromagnetics

 Not Micromagnetics                                g
                                             Coils generate ~ 200 Oe
 – Solves Maxwell’s
 Equations.
                                      1.0     10,000
                                 NiFe 1 0 T ~ 10 000 Oe
   Left side not shown.




                                       The business end

                                      2.4T CoFe
                     ABS
Field in Media Hm ~ 10,000 Oe.           Gap ~ 20,000 Oe
   MR Head: Writing A Fresh Track-                                1. Overview
   Reversing the media magnetization

                                        What’s good for a write field profile.


                                                    1. Large field at center Hm

      top                       return
      pole                       pole 2. Large dH/dx
               Writer gap
                      g p
               ~ 100 nm
                                                      Down-track direction x
      M                            M
ABS                                   HMS (Head-To-Media Separation) ~ 15 nm

                            
                                   Media ~ 10 nm magnetic layer
                      H
 Recording a transition
    8/19/97                            HSE
GMR (Giant Magneto-Resistive) Read Sensor 1. Overview
                   2007 Nobel Prize in Physics: Fert and Grünberg
                                                   1
                                                          GMR transfer curve
                                                                0.75

   ~ 300 Å                                                       0.5




                                                           2)
                                                 R (Rfull/2
                                                                0 25
                                                                0.25

                                                                   0
                                                                        0   45   90   135   180
                                                                -0.25

                                                                 -0.5


   media                                                        -0.75
                                                                                 V ~ 4mV
                                 Current flows In the
                                                                  -1
                                 Plane (CIP), mainly
                                 through Cu layer.
                                                                AFM = NiMn (High TN). 200 Å
                    Thin Films
             TEM                                                PL = CoFe (big moment). 25 Å
                                                                         Fe Permalloy 25 Å
                                                                FL = Ni80F 20 P  ll

                                 GMR effect involves surface and bulk
                                 scattering of spin polarized electrons
 ABS view                        between the Free Layer and Pinned Layer.
                                                                                                          1. Overview
The Medium: A “hard” granular magnet
                     Lubricant
                             Carbon
                             C b
                                                                                      Need ΔE >> kBT
                             Co-alloy
                              CrV
                                           CoCrPt
                              NiAl


      glass substrate

Grain size ~ 16 nm               Grain size ~ 9nm


                                                         Energy barrier ~ ( i volume)*(crystal anisotropy)
                                                         E      b i       (grain l  )*(    t l i t       )


                                                                                     Media M-H loop
                                                                                            0.0003


                                                                                            0.0002
1 Gbit/in2 media                 50   Gbit/in2   media
                                                                                            0.0001


                                                                       M                         0

                                                                           -10000   -5000             0    5000   10000
                                                                                            -0.0001


                                                                                            -0.0002


                                                                                            -0.0003         Hc
                                                                                                      H

                                                             SUPERPARAMAGNETISM: Smaller
                                                             grains are worse for thermal stability
  SNR ~ N: Smaller grains are better.                       Hc ~ (M)(anisotropy)(grain volume)
Noise sources = head electronic + media transitions                     1. Overview
     Media SNR is the biggest limiting factor to smaller bit size.
                   (Number of grains per bit): Smaller grains are better.
             SNR ~ 
                                                                         Recording
330kfci                                                      1.2G/s      process breaks
                            g                    g
                      Need: grain size << bit length                     down above
                                                             1.4G/s
                                                             1 4G/s
390kfci                                                                  1.4~1.5G/s due
                                                                         to combination
445kfci                                                      1.6G/s      of high freq
                MFM T k I
                     Track Image vs. Li Linear D it
                                               Density                   writing and
500kfci                                                      1.8G/s      high fly height.
              and Data Rate (at the fixed RPM 20krpm)
525kfci                                                      1.9G/s
                                                          D 2.0G/s AABdis d i
                                                               illustrating SNR degradation
                                                          Data ill      i
550kfci                                                                    operating
                                                          with increasing linear density at
                                                             2.2G/s 180% of it’s
608kfci                                                             designed
635kf i
635kfci                                                      2 3G/ surface
                                                             2.3G/s
                                                                    velocity.

663kfci                                                      2.4G/s

  kfci=kilo-flux changes per inch (1000 per inch)
  G/s = Gigabits/second
                                                                      1. Overview
The Trilemma of shrinking dimensions.
1.   Smaller bits require smaller media grains to maintain SNR.
2.   Smaller grains require larger anisotropy (energy barrier) to
     maintain thermal stability.
3.   Larger anisotropy requires larger write fields to switch media
     transitions.
     transitions
•    Also: Smaller dimensions lead to less responsive read elements (magentically).
•    Larger write fields require large moment materials to put at the business end of
     the it l
     th write element.t
•    CoFe at 2.4T is the largest moment material stable at room temperature and
     has been used since 1999.
Micromagnetics to the rescue (and perpendicular recording and
    tunneling magnetoresistive read elements, …).
                                          2. Micromagnetics
       g                  g
Micromagnetics of Ferromagnets: Interacting, uniformly
                                          g,         y
                  magnetized grains.

  Thin ferromagnetic films with small shapes and big
    demagnetization (magnetostatic) fields at edges.


         s                             Cartoon of a thin film
                                       C t      f thi fil




  G i size ~ 10 nm fl 1000’ of atoms.
  Grain i            1000’s f t
                                                             3. MRM
                                                     2. Micromagnetics
Landau-Lifschitz-Gilbert Equation
 Precession and phenomenological damping:
     Torque Equation
                                       Add damping 
                                                           
      dM                                                    dM ( r , t )
           M  H         H (r , t )  H eff (r , t )  
       dt                                                     dt
                  
                                      
                                                           
              2 dM
                                                   
LLG: (1   )        M  H eff       M  M  H eff  = M
                 dt                   M

                                          ~ 0.1 – 0.01
     Norm conserving:                 Spin-wave interactions, magnetoelastic
                                       p                    , g
           2                        coupling, impurities,…
     dM             dM
               2M      0
        dt            dt
            
     a  (a  b )  0
                                                                 2. Micromagnetics
LLG: Solutions for single moment M of
   NiFe in external field H=1000 Oe.
                                  
                                                     
                                                                      
                              2 dM
                                                          
                        (1   )     M  H eff     M  M  H eff
                                 dt                 M
           =0                              =0.1                             =1

               M       
                    MH            M
                                     
                                  MH
                                                                                     M

           H                                     H                             H


  Precession only
  P      i     l                   Slo relaxation
                                   Slow rela ation                          relaxation
                                                                       Fast relaxation.

   H                       For H||z and M||x:                         Use large for
f     2.8GHz               -MxH ~ y
                              MxH                                      steady-state
                                                                       steady state
   2                        -Mx(MxH) ~ z                              solution.
                      Magnetic Modeling Magnetic Films.
                   Jason Mercer and John Whitehead (MUN).
             
                                
                                                 
           dM                        
(1   2 )      M  H eff     M  M  H eff
            dt                 M


              y           g
  16x16x16 system of magnetic
  dipoles with periodic BCs on
  the side and open BC’s top
  and bottom, solved with the
                        damping.
  torque equation plus damping
  Video shows the system
  evolving towards equilibrium
  after being given a random
  initial configuration.
  Colors represent different
                      moments.
  orientations of the moments




                                John Whitehead.
                                                                                 3. MRM
                                                                         2. Micromagnetics
Interacting Grains: The “Field” in LLG
  Each grain M(r) is acted upon b an effecti e field with
                            pon by effective          ith
  numerous contributions: Heff (r)= -E/M(r)

                                                                   i b
                                                           Interacting bar magnets

                                                     
   H effective  H applied  H anisotropy  H exchange  H magnetosta tic
                 E.g., Writer   Very strong in                                    Long-Range
                                                       NN only                    demagnetization
                 head field     Media Very
                                Media.                                     NN
                                                  H ex (ri )   JS / M s  M j
                                                                   2     2
                                                                                  fields increase
                                weak in Head                                j     computational
                                materials.     Ferromagnets: Mi || Mj             demands.

  Heff = Heff[M(r)] is a functional of M(r): Self-consistent solution required.
                     
                                        
                                                                                       
                 2 dM
                                             
           (1   )     M  H eff     M  M  H eff
                    dt                 M
                                                               2. Ferromagnets
                                                            2. Micromagnetics

       g              g        g
Ferromagnets: Cannot ignore “magnetostatic” fields.
                                                        
                 M (r ' )( r  r ' )        n(r ' )  M (r ' )( r  r ' )
                                                ˆ
H ( r )    d          3              dA              3
           V          | r  r '|           S             | r  r '|
                                                   For grains of finite size,
       Long range interaction ~          1/rn      it’s not just dipole-dipole
   Increases computational demands.
                        M2     Adjacent bar magnet lies anti-parallel


                                   • Outside of bar M=0; H = B = stray field
       S               N           • Inside of bar B = H + 4  Mr,
                                    ‘demagnetizing’ field H opposes M
                                                                       2. Ferromagnets
                                                                    2. Micromagnetics
Shape Anisotropy: Simple Argument

  • Dipole-dipole interaction (first terms in multipole
    expansion)
                                      
                         mi  m j 3(mi  rij )(m j  rij )
E=-MH         E 3 
                    pairs r ij            r 5 ij
                            ij

    • Consider energy of dipole 1 (lattice spacing                  a=rij)
                        a3E
                           2
                              (cos 12  cos 13  cos 14  cos 15 )
                3        m
        2       1   4
                         3(cos 1 cos  2  cos 1 cos  4  sin 1 sin  3  sin 1 sin  5 )
ˆ
y                                                                   Put i  
                5                 4  3(2 cos2   2sin2 )  2

                       Completely isotropic.
            ˆ
            x
                                                                2. Ferromagnets
                                                             2. Micromagnetics
Shape Anisotropy: simple argument


                g (         )
     • Create edge (remove 3)

                         a3E
         2   1   4
                            2
                               (cos 12  cos 14  cos 15 )
                          m
             5
 ˆ
 y                        3(cos 1 cos  2  cos 1 cos  4  sin 1 sin  5 )

                                                  Put i  
                          3  3(2 cos2   sin2 )  3  3cos2   3sin2 
         ˆ
         x
         Uni-axial i t        induced b edge.
         U i i l anisotropy i d      d by d
         Energetically favorable for spins to align
         parallel to the edge and to neighboring spins.
                                                             2. Ferromagnets
                                                          2. Micromagnetics
Patterned Devices: Shape Anisotropy
   • Series of solutions for platelets with different aspect ratios:
     what do we see?
       – CoFe, 2.5 nm thick, 10 nm cells (single layer)


  Micromagnetic Modeling




  M    t t d t li         ll l t      d
 •Moments tend to lie parallel to an edge.
 •Corners are complicated.

                       Steady State Solution
                                                               2. Ferromagnets
                                                            2. Micromagnetics
GMR spin valve response to a media transition field.
                          H         Field from media transition decays quickly
         Quiescent state (H=0)        y             Response to field from a media
                            ABS
  transition


                 FL                                         FL

               ABS                                       ABS
                 Shape anisotropy limits response for smaller devices.
                 Sh      i t      li it           f      ll d i
                                  DV=1mV,
                PL                not 4mV.
                                                            PL
               Thermal Fluctuations.                                          4. Spin
                                                                      2. Micromagnetics

  G d o eC oc             so          o ccou o        e       uc u o s.
LLG and Monte Carlo can also be used to account for thermal fluctuations.



                                                                                 temperature
                                             Thermal averaged magnetization vs temperature.
     2D square lattice with                      2D square lattice. J=K=1 (Ising model).

                                       1.2
     strong axial anisotropy.
       · · · · · · · · ·                1                                               LLG
                                   M                                                    MC
     · · · · · · · · ·
                                       0.8
   · · · · · · · · ·
                                       0.6
                                       06


   •Temperature fluctuations           0.4
   reduce the thermal average
   magnetic moment <M>.                0.2


   •This leads to thermal noise,        0
                                             0        0.2       0.4         0.6      0.8       1
   reduction of AF/F pinning, …
                      p     g,                                        T/J

                                                                            Transition
                                                                            Temperature
More fun with Micromagnetics                   2. Micromagnetics

     Spin waves in thin films      Media M-H Hysteresis Loops
      - finite temperature         - finite temperature
      - damping                    - sweep rates (VSM very slow,
                                     Disc Drive very fast)




              Magnetostatic mode




    Theory vs LLG for 3
    ferromagnetic layers. ky=0.
    g/J 0.2,    0.15
    g/J=0 2 Hy=0 15
                                                                                                       3. MRM
       3. Micromagnetic Recording Model




                                      2) Write on LLG     3) Generate field from
   1) Finite Element Method
                                                             transition.
   (FEM-commercial                    micromagnetic
                                                             Perform read back with
   software) write head field         model of medium.
                                             f   di          micromagnetic reader model.

                                      •Use to write single-tone
                                                                                     0.8


                                      patterns.
                                      patterns                                       0.6
                                                                                     06

                                                                                     0.4

                                                                                     0.2




                                                                      Voltage (mV)
                                      • Pseudo-random sequence
                                                                                        0
                                                                                         0.45   0.55   0.65        0.75   0.85   0.95
                                                                                     -0.2

                                                                                     -0.4



                                      require very long patterns                     -0.6

                                                                                     -0.8




                                      (computationally intensive).                                     down track (um)




2D contours of a write field at the
media plane
Longitudinal Recording: M in the plane of the disc.     3. MRM
Media Transitions: Small noise on top of Big noise.
                  bits    i h
          343000 bit per inch = 74 nm

                                         2D in-plane anisotropy

                                         64x64 grains



                                             Where s
                                             Where’s the
                                             transitions ?
                                                       3. MRM
Longitudinal Media Transitions.
                     343000 bits per inch

                                                  g     g
                                       Alternating average
                                       magnetization between
                                       transitions

                                       Down track component only:

                                             bright

                                             dark
     signa voltage
         al
                                                              3. MRM
  Medium Orientation Effects
• Orientation Ratio (OR) accounted for by assuming a
  Gaussian distribution for the media grain anisotropy
  axes di ti HK: mean and St d d Deviation.
       direction               d Standard D i ti

       z
                                            
              x


     Isotropic media – very large standard deviation
     Oriented media – ‘smaller’ standard deviation (SD).
                               p           performance for media
                             Expect better p
                             with a preference for M to lie in the
                             down-track direction.
                                                             3. MRM
100Gb/in2. Recorded tracks at high kbpi much
       improved with oriented media.
 Isotropic media.       Oriented media.          OR ~ 1/SD
290 and 640 kbpi.
 .                          640 kbpi.              Amplitude vs OR




 290 kbpi               OR=1.23         OR=1.6
            640 kbpi




             g                y         g              p
A clear message was received by recording media development
groups. Highly oriented media is now the industry standard.
                                                                                 3. MRM
Micromagnetic Calculation of Signal-to-transition-noise ratio
   Single-tone pattern.                          L
                                                                           
                                                    V ( x)  dx
                                                        2
                                                                          
 Bertram formula:         SNR =   10 log L
                                        
                                                  0                        
                                                                              = Amplitude
                                           V 2 ( x)    V ( x)  2 dx          i
                                                                                  Noise
                                        
                                        0                                 
                                                                           

        Average over 50 random configurations of the medium
        A                   d       fi    ti   f th    di
        anisotropy direction and magnitude.
    g
  Magnetization Profile     430 kfci, 100 ktpi
                             30 c , 00 tp                               g
                                                         Read-back Voltage




  Better SNR is the key driver for higher areal density
                                                                                             3. MRM
Effects of Medium Anisotropy Hk distribution
                                                                               640 kbpi
                                                                     Hkstd=0                   V
                                               Hk
                                                                                               dV/dx
                SNR vs Hk std dev.
                F ll Micromagnetic calculation of Transition
                Fully Mi          ti   l l ti     fT     iti      Hkstd=10%
                                                                  Hk d 10%
                  SNR vs Hk distribution. TPWG=0.2 um.
                       Hk = 9000 Oe. 430 kbpi. 100 ktpi.
           17
           16
           15
           14
SNR (dB)




           13
           12                                                     Hkstd=20%
           11
           10
            9
            8
                 0        500        1000       1500       2000
                                 Hk_std (Oe)


                                                               A clear message: Distributions in media
                                                               properties are bad for performance.
                                                                                        3. MRM
 Medium Grain Size Impact on Single-tone SNR vs kfci.

            MRM for Yellowstone 180 ktpi. Effect of media
            grain size on modeled single-tone media SNR.
           17                                                     Huge benefit from using
           16                                                     smaller mean grain size
           15                                                     on SNR, especially at
           14
                                                                                density.
                                                                  higher linear density
SNR (dB)




           13
           12
           11                                                               Data illustrating SNR
           10         10 nm                                                 degradation with
            9                                                               increasing linear density
                      8 nm
            8
                      6 nm
            7
                300     400        500        600           700
                                    kfci


                                                                      Pseudo-random
                                                                      sequence
                                                  4. What’s New
     Comparison of Longitudinal & Perpendicular Recording
               Longitudinal Recording                                                                       Perpendicular Recording

                                                 BL                                                                                                                  BL




L
                                                                                                                                                                                                  P
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         g
      Magnetostatic fields                                                                           Magnetostatic fields
      destabilize the transitions                                                                    stabilize the transitions.
      (superparamagnetism).
                                                                                                4. What’s New
Larger Write Fields due to media soft underlayer.
   •   1
            Longitudinal Recording                         •       1
                                                                                      Perpendicular Recording



                                      Deep Gap Field HG

                                                                                                                Auxiliary
                                                                          Main Pole                             Pole


                                                      L       P


                                                               t
                                      Field in Medium HM       K                      Media Soft Under Layer
                                      (Fringing Field)                                                           Double
                                                               Keeper,   100                                   Layer
                                                                                                                 Medium
                                                               Deep Gap Field HG



  Single layer medium (longitudinal or perpendicular):             Double layer perpendicular media with soft magnetic
  • Transition is recorded by fringing field                       Underlayer: media becomes part of the write element
                                                                   • Transition is recorded by deep gap field
                                            4. What’s New
…and more field emanating from media transitions


   GMR read sensor




                      Stronger fields from perpendicular
                      St        fi ld f          di l
                      bits = larger play-back amplitude.
                                   boost
                      Another ~50% boost.
                   And…Tunneling GMR Heads: 4. What’s New
                  Double the read-sensor amplitude.
                  (
•New Structure aka: TMR, MTJ, SDJ,Tunnel valve, spin tunneling head)
•New materials                                     TOP SHIELD / ELECTRODE
 N    h i
•New physics
   Energy scale                       STABILILITY LAYER                                      GAP ELECTRODES
                                      FREE LAYER                                          TUNNEL BARRIER
                                      SAF PINNED LAYER

                                               BOTTOM SHIELD / ELECTRODE
  ield
Shi




                                                                                                   Spin direction
                               Tunnel                                  barrier
   eld




                                                                                                    is conserved
                                              Incident electron
Shie




                               barrier                                                            during tunneling
                                              wave function           height


         CPP          TEM: ABS view                                                          Transmitted
                                                  Ferromagnet     Al2O3     Ferromagnet      electron
                                                       1          barrier
                                                                                2            wave function

CPP = Current Perpendicular to Plane
                                                                   Distance
            5. What’s Coming




Then What
Th Wh t ?
                                                                        5. What’s Coming
ECC - Exchange Coupled Composite Media
                                                      Happl
                                                                            Soft: low Hk

• Magnetic hard layer (high Hk) provides the
thermal stability                                                           Hard: high Hk
• Magnetic soft layer (low Hk) functions
 as a ‘lever’ to help switch the hard layer during
writing.                                                      Hexc
                                                             g                  g
                                                     Switching Field vs Field Angle

          Victora and Shen (2004)




        Eric Meloche
        February 14, 2008
                                                                                                    5. What’s Coming
HAMR – Heat Assisted Magnetic Recording

                                                                                       Drive
                                                                                    Temperature




                                                                    civity
                                                               C oerc
                                                                                                            Heat
                                                                                  Store                     Media
                                                                                  Here
                                                                                                    Cool
                                                                                                    Media



                                                                             Available Head Field                   Write
                                                                                                                    Here

                                                                                                            Temperature


  • The challenge in making HAMR work is in creating sufficient thermal gradients that will prevent interference between
  adjacent bits.




     Eric Meloche
     February 14, 2008
                                               5. What’s Coming
               Patterned Media
 Lithographically Patterned Bits, single magnetic grain to
                      j
  decrease transition jitter and increase SNR.




               diameter C /C bil
        100 nm di                   dots ith           i d
                    t Co/Cu bilayer d t with 200 nm period
                                                                   5. What’s Coming
Or…
    Self-Assembled                       Arrays.
    Self Assembled Magnetic Nanoparticle Arrays
An illustration of nanoparticle self-assembly via solvent evaporation.



                                        Solvent
                                        Evaporation




 Fig.9.11. TEM images of (a) a 2D
 assembly, of the 8 nm cobalt nanoparticles
 on an amorphous carbon surface

   Co or FePt.
       Collaborators at Seagate:                     Collaborators at MUN:
   Johannes van Ek (now at WDC),                  John Whitehead, Jason Mercer,
                        Heinonen
  Eric Meloche and Olle Heinonen.                       Trinh Nguyen




Physical Oceanography
Soft Matter
Biophysics
Polymers
Photonics
Magnetism
Sensors and Actuators
Computational Science
                                   http://www.mun.ca/physics/
Environmental Science
                    Magnetic Materials
                                • Carbon 3-4nm
                                • Magnetic layer ~20nm (CoCrPt)

                                 Interlayers
                                •I    l

                                • SUL 80-200nm




Eric Meloche
February 14, 2008
                       Writer Risetimes
• Goals: Determine the most important factors that limit flux rise
times throughout the entire pole structure
                                          Simple Pi t
                                          Si l Picture
                             • Perpendicular component of magnetization in the interior of
                             the writer has a fast response to coil field
   AB C
                            Points B and C - Fast response time
                           • Magnetization response slows down near the ABS
                                 Point A - Slow response time
                                      Time resolved optical measurement of
                                      Pole    i         ti ti
                                      P l region magnetization

                                             Coil current

   Eric Meloche
   February 14, 2008       M.R. Freeman et al., J. Appl. Phys. 81 (1997)
Micromagnetics: Interacting grains.                                       2. Micromagnetics

Exchange in terms of spins:
                                                                 NN
            E ex   J ij S i  S j   JS / M s                M       Mj
                                                     2       2
                                                                      i
                       j
                      ij                                         ij
                                                                  j
                    atoms                      J’’           grains


    MA=(MS/S)(S1+S2)
       (    )(               A    1        2         3   4       B       (M
                                                                      MB=(MS/S)(S3+S4)

                                      J’
    Eex = -(J’S1 S2 + J’S3 S4 + J’’S1 S3 + J’’S1 S4 + J’’S2 S3 + J’’S2 S4)

Exchange in terms of grains:                         a constant

                  Eex = Eex(intra-grain) - J’’(S2/Ms2)MA MB
                                     J’
This approximation works provided J is strong enough so that S1 || S2.
 Magnetic grain size is determined roughly by the “exchange length”
                             ~ 5 – 15 nm.
                  p     p                g
       Model for Spin Dependent Scattering
             Density of States for Transition Metal Electrons
                                          Energy

                    Empty                           M
                    States     4s                        4s

                                          3d
                                                              Fermi
                    Filled
                    States                     3d




minority carriers (spin antiparallel to        majority carriers (spin parallel to M)
M) can scatter into more unoccupied            can scatter into fewer unoccupied
states giving them higher resistivity.         states giving them lower resistivity.
LLG media M-H loop.                                                                             3. MRM

  Anisotropy axes of grains are randomly oriented in plane:
• A i              f    i          d l     i    di l
  2D isotropic media. Hk=6400 Oe. Hc ~ Hk/2
                   1495_2: fac=0.055, Ms=350, Hk=6400, exh=0

                                         1
                                       0.8
                                                                 M || H
                                       0.6
                                       0.4
                                       0.2
                    H
       M/Ms




                                         0
                                                                           H
       M




              -1                       0
                     -0.8 -0.6 -0.4 -0.2 2 0
                                      -0.2     0.2   0.4   0.6   0.8   1
                                      -0.4                                               Media M-H loop
                                      -0.6                                                      0.0003



                                      -0.8                                                      0.0002



                                         1
                                        -1                                                      0.0001




          M || H                        H/Hk
                                                                           M
                                                                               -10000   -5000
                                                                                                     0


                                                                                                -0.0001
                                                                                                          0   5000   10000


                                                                                                -0.0002


                                                                                                -0.0003




M2=Mx2+My2.
   M                                                                           Expt.
                                                                               Expt                       H




Mx=My=M/2                    H=0
  How a writer works.                                        2. Ferromagnets
                                                          2. Micromagnetics


            H ~ 2000 Oe
                                 H ~ 200 Oe
                BP                                      Side View
    gap
                       Coils
~ 20,000 Oe                        x


                     g
              •Coils generate about 200 Oe.
      ABS •Field at the write gap is about 20,000 Oe.
              •How do we get a 100-fold increase in field strength ?

                                                          M ~ Cos 

                   g
          Soft Magnetic Material means that M can                      H
          easily align with the applied field H.
                                                         2. Ferromagnets
                                                      2. Micromagnetics
Field from coils rotates the magnetic moments

              Top-down view                     •Each magnetic grain
        I=0                   I >> 0            acts like a small bar
                                                magnet (millions of
                                                them).
Top                                             •The stray field from
Pole                                            each bar
                                                H ~ 2Bs
                                                acts on neighboring
                                                   i      h l li
                                                grains to help align
                                                them further.
                H=0                    H~2000
 ABS                                               Pole tip t i l C F
                                                   P l ti material: CoFe
                                  H ~20,000        2.4T = 24,000 Oe
                                                           1. Overview
Bits of Information: Pseudo-Random Sequence.


  •Information recorded on the medium in a disc drive appears as a
         sequence
  random sequence.
  •Meaningful detection of this data (SNR) is limited by bits that
  are close together: The high bpi portion of the sequence limits
  data recovery.




         High Resolution M ti F     Mi
         Hi h R l ti Magnetic Force Microscopy (MFM)

				
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